Jean-Baptiste-Antoine Lassus (19 March 1807 – 15 July 1857) was a French architect who became an expert in restoration or recreation of medieval architecture. He was a strong believer in the early Gothic architecture style, which he thought as a true French and Christian tradition, and was opposed to the classical Graeco-Roman styles promoted by the academic establishment.

Lassus took commissions to design churches and extensions to convents.
In 1835 he began to restore Saint-Séverin, Paris.
In 1836 Lassus and Félix Duban were given the task of restoring Sainte-Chapelle.
He worked on this project for the rest of his life, concentrating on the spire and the decorations of the interior, which he restored to their earlier form.[2]
In 1837 he was named architect for the restoration of Saint-Séverin.
In 1839 Lassus and Étienne-Hippolyte Godde were given the task of restoring Saint-Germain l'Auxerrois.
In 1840 Lassus was asked to undertaken construction of the Basilique Saint-Nicolas de Nantes.
In 1841 he built the tomb of the Abbé Charles-Michel de l'Épée in Saint-Roch, Paris.[1]

Lassus contracted a disease of the liver, which he neglected.
He died on 15 July 1857 in Vichy, where he had gone in search of a cure.[4]
His funeral was held in Notre-Dame on 20 July 1857. His tomb was designed by Viollet-le-Duc.[1]

Lassus designed a few secular buildings, where he showed that he could build in fashionable Louis-Philippe or Louis XIII styles as required by his client, or for harmony with the surrounding buildings.
However, he mostly worked on Gothic-style religious buildings, notably Saint-Nicolas de Nantes (1840), Sacré-Coeur de Moulins (1849), Saint-Pierre de Dijon (1850), Saint-Jean-Baptiste de Belleville (1853) and the Église de Cusset (1855).[5]

Lassus was critical of the Académie française, which would only recognize pagan Greek and Roman architecture, which he saw as foreign imports.[2]
He said of Greek architecture that it did not suit the French religion or climate, or even the materials available in France.[6]
He strongly favored Gothic architecture, a Christian style that had originated in France.
He was a purist, would only use historical building materials and tried to remain true to original concepts.[2]
He believed that the buildings of the first Gothic period were rational and functional, the peak of French architecture. Later Gothic had degenerated and Renaissance architecture introduced foreign and pagan influences. He considered that restoration of Gothic buildings must respect their formal and structural authenticity.
The principles of the first Gothic period would be the basis for a new 19th century architecture.[5]